This invention relates to an improved process for preparing asymmetric isoindoline pigments having high brilliance and color strength and good light- and weather-fastness.
Compounds of formula I are valuable organic pigments previously described in the literature. Such compounds are disclosed, for example, in U.S. Pat. Nos. 3,991,054, 4,166,179, 4,316,023, and U.S. Pat. No. 4,401,815, and in Canadian Patent 1,140,130. These references, however, disclose processes that require an initial preparation of 1-amino-3-iminoisoindoline (also known as 1,3-diiminoisoindoline) (formula A) ##STR2## or ring-substituted derivatives thereof from 1,2 -dicyanobenzene and ammonia. Cf. F. Baumann et. al., Angew. Chem., 68, 133-150 (1956). Among the technical and economical disadvantages of these methods is the need to convert 1,2-dicyanobenzene into this intermediate in an additional step when compared to the s process of the present invention. In addition, the need to handle ammonia causes safety and environmental problems.
The use of dialkoxy isoindolines instead of 1-amino-3-amino-isoindoline avoids the need for ammonia. For example, U.S. Pat. No. 4,500,718 discloses a method using alcoholate bases (formed in situ from a strong base in an alcohol) to convert 1,2-dicyanobenzene into intermediate mixtures analogous to those of formula VIII (see below). The disclosed method, however, was used to form only symmetric bis(cyanomethylene) compounds.
U.S. Pat. Nos. 3,646,033 and 3,794,659 disclose a stepwise method for using 1-amino-3-amino-isoindoline, dialkoxy isoindolines, and other such reactive isoindoline derivatives to prepare asymmetric isoindoline derivatives. These patents, however, teach the importance of removing impurities from isolated intermediates and thus, do not suggest methods in which intermediates are not isolated. In addition, these patents exemplify the preparation of compounds different from those of the present invention.
Therefore, an object of the present invention was to develop a procedure for synthesizing isoindoline pigments of formula I in pure asymmetric form and without the use of ammonia and 1-amino-3-amino-isoindoline and without the need to isolate intermediates.